Add junos_system declartive module and other related change (#25859)

* Add junos_system declartive module and other related change

*  junos_system declartive module
*  integration test for junos_system
*  integration test for net_system (junos platform)
*  pep8 fixes for junos modules
*  move to lxml from elementree for xml parsing as it support
   complete set of xpath api's
*  other minor changes

* Fix CI and doc changes

* Fix unit test failures

* Fix typo in import

* Fix import issue for py2.6

* Add missed Element in import

lib/ansible/modules/network/junos/junos_user.py

@@ -91,6 +91,11 @@ options:
     required: false
     default: present
     choices: ['present', 'absent']
+requirements:
+  - ncclient (>=v0.5.2)
+notes:
+  - This module requires the netconf system service be enabled on
+    the remote device being managed
 """
 
 EXAMPLES = """
@@ -116,13 +121,16 @@ RETURN = """
 """
 from functools import partial
 
-from xml.etree.ElementTree import Element, SubElement, tostring
-
 from ansible.module_utils.junos import junos_argument_spec, check_args
 from ansible.module_utils.basic import AnsibleModule
 from ansible.module_utils.junos import load_config
 from ansible.module_utils.six import iteritems
 
+try:
+    from lxml.etree import Element, SubElement, tostring
+except ImportError:
+    from xml.etree.ElementTree import Element, SubElement, tostring
+
 ROLES = ['operator', 'read-only', 'super-user', 'unauthorized']
 USE_PERSISTENT_CONNECTION = True